1. Field of the Invention
This invention relates to ferritic stainless steel sheets having excellent deep-drawability and surface smoothness applicable to home electric appliances, kitchen appliances, construction, and automobile components and to methods for making the same. In particular, the invention relates to a ferritic stainless steel sheet suitable for use in automobile fuel tanks and fuel pipes which are made by high deformation such as deep drawing and pipe expanding, and are highly resistant to organic fuels such as gasoline and methanol which contain organic acids produced in the ambient environment. A method for making the same is also provided.
2. Description of the Related Art
Ferritic stainless steels which do not contain large amounts of nickel (Ni) are cost effective compared with austenitic stainless steels and a-re free of stress corrosion cracking (SCC). Due to these advantages, ferritic stainless steels have been used in various industrial fields. However, known ferritic stainless steels exhibit low elongation of approximately 30% and are thereby inferior to austenitic stainless steels, for example, SUS 304, in workability. Known ferritic stainless steels do not have sufficient workability for high deformation such as deep drawing, and typically, press forming, and are not suitable for mass production. Because of these problems concerning formability, the use of ferritic stainless steel in various fields such as automobiles, construction, and home electric appliances has been severely limited.
Several attempts have been made to improve the formability of ferritic stainless steels. Among these, Japanese Unexamined Patent Publication No. 3-264652 proposes optimization of manufacturing conditions of ferritic stainless steels containing Nb and Ti in order to obtain an aggregation structure of 5 or more in X-ray intensity ratio (222)/(200) and to improve the formability.
In this technology, however, the revalue is only about 1.8; hence, application to fuel tanks requiring complex forming by deep drawing and to fuel pipes requiring pipe-expansion and bending is difficult. Moreover, even if applied at all, defect rates are high and mass production is not practical. On the other hand, ternesheets, i.e. soft steel sheets provided with plating containing lead, have been widely used as the material for automobile fuel tanks. However, regulations on the use of lead are becoming stricter from an environmental point of view and substitutes for the ternesheets have been developed. The substitutes developed have the following problems. Lead-free Al—Si based plating materials are unreliable in terms of weldability and long-term corrosion resistance and the application thereof is thus limited. Resinous materials have been applied to fuel tanks, but since these materials naturally allow minute amounts of fuel to permeate, the industrial use thereof is inevitably limited under fuel transpiration and recycling regulations. Use of austenitic stainless steels which can be used without lining have also been attempted. Although austenitic stainless steels are superior in formability and corrosion resistance to ferritic stainless steels, they are expensive for use in fuel tanks and may suffer from stress corrosion cracking (SCC). Thus, the use of austenitic stainless steels has not been practical.
In such a situation, enormous advantages such as improvement of the global environment can be achieved if these materials can be substituted by ferritic stainless steels which are recyclable.
Since the revalue of ternesheets is approximately 2.0 ferritic stainless steels must attain an r-value of 2.0 or more for them to replace the ternesheets. Ferritic stainless steels must also have long-term corrosion resistance to deteriorated gasoline containing organic acids such as formic acid and acetic acid which are formed in the ambient environment in order for the ferritic stainless steels to be applied to fuel components such as automobile fuels tanks and pipes. However, no investigation has specified suitable compositions for attaining these goals.
As previously described, the r-value of the known ferritic stainless steels is only approximately 2.0 at most, and application of ferritic stainless steels to pressed components requiring extensive deep drawing has not been achieved. Another problem with ferritic stainless steels is the generation of rough surfaces after pressing by deep drawing. Here, rough surfaces include the orange peel condition caused by rough crystal grains and the presence of corrugations aligned in the rolling direction (L direction) as a result of cold rolling thereby rendering undulating surfaces in the sheet width direction.